The world number one women's tennis player, Monica Seles, has been stabbed in the back during a quarter-final match in Hamburg.

The 19-year-old American star was rushed to hospital with a wound half an inch (1.5cm) deep in her upper back. Doctors said her injuries were serious, but not life-threatening.

"She was very lucky," said the tournament doctor, Peter Wind. "Neither the lungs nor the shoulder blades were affected. Monica is still suffering from shock, and will stay overnight for observation."

There was immediate speculation that the attack was politically motivated because of Monica Seles's Serbian roots. She is known to have received death threats in connection with the Yugoslav conflict.

But Hamburg police were quick to rule this out. They described her attacker as a 38-year-old man from East Germany, and said he appeared confused and may be mentally disturbed. Later reports suggested he may have been a fan of tennis rival Steffi Graf.

In full view of crowd

The attack happened during the rest break in the match, against Bulgarian player Maggie Maleeva. Miss Seles was leading 6-4, 4-3 when she took a rest on her courtside seat during the changeover.

A man described as stocky and balding leaned over the three-feet-high (91 cm) barrier and stabbed her from behind. Miss Seles let out a scream, clutched her back and stumbled on to the court.

The attack took place in full view of the 6,000-strong crowd watching the match.

"He held the knife with both hands as he stabbed her in the back," said one eyewitness.

The umpire, Stefan Voss, ran from his chair for ice and a towel.

"Nobody saw him coming," he said. He described the 10-inch (25cm) long knife, which fell on the floor following the attack, as "a kitchen knife, like the sort you cut meat with. It was very sharp."

Monica Seles is expected to be out of action for about four weeks, missing the Italian Open, which begins on Monday, and the French Open on 24 May, in which she was to have been the defending champion. Her participation in Wimbledon in June is also in question.